Vehicles have been developed to perform engine stop at idle conditions when specific conditions are met and then to automatically restart the engine when restart conditions are met (also referred to as start/stop systems). Such idle-stop systems enable fuel savings, reduced exhaust emissions, reduced vehicle noise, and the like. Similarly, hybrid electric vehicle systems operate a vehicle via an engine during selected conditions and via an electric motor during other conditions. The reduction in engine operation time enables significant fuel savings.
However in such vehicle systems, large torque pulsations may be experienced during engine restarts when the engine is being cranked, as well as during engine shutdowns when the engine is spinning to rest. The torque pulsations may be due to compression/expansion work in individual cylinders. In addition, engine cranking requires significant torque and power to overcome the peak pressures, partly because the engine does not have much rotational inertia as compared to higher engine speeds. To reduce the NVH and engine performance issues associated with such torque pulsations, various approaches have been developed to reduce the effective compression ratio of the engine during engine start and shutdown events.
One example approach is shown by Gibson et al. in U.S. Pat. No. 8,352,153. Therein, intake valve closing (IVC) timing is retarded during an engine shutdown and/or during a restart for a cylinder where fueling is resumed during the restart. The resulting delay in the start of compression reduces the maximum cylinder pressure, which reduces the power required to overcome the cylinder pressure and also reduces the associated torque pulsation. Another example is shown by Gibson in U.S. Pat. No. 8,412,443. Therein air charge may be controlled during an engine shutdown via a throttle or alternate charge control device so that it does not exceed a level causing a compression torque that may stop the engine before a restart. In still further examples, such as in hybrid vehicle systems, reserve power may be stored during an engine shutdown to overcome the torque pulsations during the subsequent crank.
However, the inventors herein have recognized potential issues with such systems. As one example, use of reserve power to overcome engine crank torque pulsations reduces the total power available to vehicle wheels. As such, this reduces the maximum vehicle speed and maximum power attainable before the engine must be started. Due to the limited power capability of the electric motor, the engine may be restarted more frequently, such as at lower vehicle speeds and lower driver demands, resulting in a drop in fuel economy. As another example, even with an aircharge level adjusted during the shutdown, there may be noticeable and objectionable NVH due to a driver change-of-mind engine restart. If the engine is kept running to reduce the NVH at a change-of-mind engine restart, the reduced engine shutdown frequency may result in a drop in fuel economy. As yet another example, the starter motors typically used to address torque issues in start-stop systems are bigger, heavier, and costlier, adding to component cost, complexity, and fuel usage.
In one example, the issues described above may be addressed by a method comprising: in response to a hybrid engine shutdown or restart event in a hybrid vehicle, actuating a cam actuator while pulling down or pulling up an engine to operate one or both of an intake valve and an exhaust valve according to an adjusted valve lift profile distinct from an unadjusted valve lift profile used during cylinder combustion, the adjusted valve lift profile enabling a lower cylinder compression pressure than the unadjusted profile; and selecting the adjusted profile based on a state of charge of an energy storage device. The hybrid engine shutdown or restart event in the hybrid vehicle may occur automatically without input from the driver, and without a change in vehicle state or a key state. In this way, unique valve lift profiles may be advantageously used to reduce cylinder pressure and minimize torque pulsations on engine shutdown and restart events.
As one example, during an engine pull-down or pull-up event in a hybrid vehicle system (such as when a hybrid vehicle is in motion and the engine is being shut down or restarted, respectively, while the vehicle continues to be propelled), a selected valve lift profile may be applied for one or more of the intake valve and the exhaust valve. The selected valve lift profile may be distinct from, and applied in addition to or instead of, a default valve lift profile applied during cylinder combustion. The selected valve lift profile may be implemented in each engine cylinder during the engine restart/shutdown event via one or more of cam profile switching mechanisms, electromagnetic valve actuators, electrohydraulic valve actuators, etc. In one example, the selected valve lift profile may be a first profile that delivers an additional exhaust valve event during a compression stroke (in addition to an exhaust stroke exhaust valve event) and an additional intake valve event during an expansion stroke (in addition to an intake stroke intake valve event). In another example, the selected valve lift profile may be a second profile that holds one or more valves in each cylinder open at a constant lift through all strokes of an engine cycle, the constant lift smaller than a peak lift applied during the default valve lift profile. In yet another example, the selected valve lift profile may be a third profile that holds the one or more valves in each cylinder open with a fluctuating lift through all strokes of an engine cycle, the fluctuating lift having a peak in the middle of each stroke, the peak lift being smaller than the peak lift applied during the default valve lift profile. In still a further example, the selected valve lift profile may be a fourth profile with a fluctuating lift that is not reduced at BDC positions. A controller may select between the different profiles during the engine restart/shut-down event based on one or more parameters such as an energy storage device state of charge (of an energy storage device coupled to a motor of the hybrid vehicle system), engine torque actuator constraints (e.g., intake throttle position), piston valve clearance, etc. Further still, distinct profiles may be selected for engine shutdown events relative to engine restart events. For example, when the piston valve clearance is smaller, the fourth valve lift profile may be selected. As another example, when the energy storage device state of charge is lower, one of the other alternate valve lift profiles may be selected.
In an alternate example, the engine system may be operated with only two valve lift profiles including a default valve lift profile applied during regular cylinder combustion plus one alternative profile for engine shutdown and restart events. Herein, the alternative profile to be used may be pre-selected based on actuator design issues and other constraints. The controller would simply choose when to use the default (e.g., normal) valve lift profile and when to use the alternative valve lift profile. In doing so, the control complexity and component requirement of the engine system is reduced.
In this way, cylinder pressure may be reduced during engine cranking and engine shutdown, thereby reducing torque pulsations and associated NVH. Another technical effect of reducing the cylinder pressure during a restart is that the power required to overcome the cylinder pressure is reduced, allowing for a larger portion of a vehicle's reserve power to be applied towards wheel torque. As such, this reduces the frequency of engine pull-up events in a hybrid vehicle. By enabling smoother shutdowns and restarts, NVH associated with a driver change-of-mind restart is reduced, and a hybrid vehicle may be “sailed” (or coasted) with the engine off for longer durations. Consequently, fuel economy is improved. By relying on one or more alternative valve lift profiles to reduce the cylinder pressure during the engine pull-up events, a smoother cranking may be achieved while relying on a smaller, lighter, and more cost-effective starter motor. Overall, the quality and repeatability of engine shutdowns and restarts may be significantly improved, while also improving vehicle fuel efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.